This invention relates to papermaking, and more specifically to improvements in shoe press belts and in the methods of their manufacture in order to achieve uniform hardness and improved durability.
A shoe press is an apparatus used to squeeze water out of a web of pulp in the pressing stage of a papermaking machine. There are two types of shoe presses: open and closed. The open type shoe press takes up a large amount of space and has the drawback that it diffuses oil. Therefore, the current trend is toward the use of the closed shoe press.
In a closed-type shoe press, a shoe press belt passes between a press roll and a shoe. A pulp web containing water is sandwiched between upper and lower felts, which pass between the shoe press belt and the press roll. Water contained in the web is squeezed out, and transferred to the felts.
A conventional shoe press belt is typically composed of an endless layer of a high molecular weight elastic substance, and is reinforced by a base member, typically a woven fabric. The circumferential length of the belt is relatively short, and consequently its working conditions are severe. There has been a need for improvement in the durability of such belts.
Various proposals have been made for improving durability. According to one proposal, which will be discussed hereinafter in greater detail, a base member of a shoe press belt is formed by winding a narrow, belt-shaped member in a helix-like spiral in which successive layers overlap but are axially displaced from one another. The base member is covered by polyurethane, which is cured, cut to a desired thickness, and grooved.
The spirally wound base member is unable to impart adequate strength to the belt in the sideward (axial) direction, and consequently, the belt is likely to stretch sideward. In addition, the winding of the narrow belt-shaped member is time-consuming, and it is difficult to control the overlap of its successive layers. Hardness of the resulting belt may be affected, and its service life may be shortened as a result.
It is an object of this invention is to address the above-mentioned problems. More specifically, one object of this invention is to provide an improved shoe press belt having overall uniform hardness and improved durability. Another object of the invention is to simplify the manufacture of shoe press belts.
A preferred shoe press belt in accordance with the invention comprises a cylindrical, elastic member formed of a high molecular weight resin and having a base member embedded therein. The cylindrical member has inner and outer walls coaxially disposed about an axis and uniformly spaced from each other radially. Every point on the starting end of the belt is adjacent to a point on the terminal end. Thus, the cylindrical member has a uniform thickness. The base member embedded in the high molecular weight resin is located between the inner and outer walls of the cylindrical member and comprises a belt-shaped member spirally wound in multiple layers which are substantially coextensive axially. The belt-shaped member by itself has the ability to resist both lateral and longitudinal stretching. By virtue of the above structure, the shoe press belt will exhibit uniform hardness over substantially its full width.
Preferably, the belt-shaped member comprises a woven fabric impregnated with the same resin as the resin of which the cylindrical, elastic member is formed. The use of the same resin for both purposes produces a strong bond not only between the layers of the belt-shaped member, but also between the wound belt-shaped member and the cylindrical, elastic member.
The shoe press belt is preferably made by the steps of winding a belt-shaped member, in multiple, substantially axially coextensive layers, onto a support body having a cylindrical surface, and forming, from a resin, a cylindrical, high molecular weight elastic member on the base member so that the base member is embedded within the wall thickness of the elastic member. A belt having uniform hardness uniform hardness over its width is thus manufactured relatively easily.
Preferably the belt-shaped member is wound from a woven fabric already impregnated with the same resin from which the elastic member is to be formed. Thus good inter-layer integration in the base member, and good integration between the base member and the elastic member, are achieved easily.